This invention relates to a process for the stabilization of the molecular weight of irradiated sodium carboxymethyl cellulose (hereinafter referred to as carboxymethyl cellulose or as CMC). More specifically it relates to a method for preparation of stabilized low and medium molecular weight CMCs which are useful in preparing solutions of low and medium viscosities.
CMC is widely used as a viscosifying compound in aqueous systems. To accommodate the large variety of applications requiring different viscosities, a wide variety of molecular weights of CMC is marketed. The viscosity of an aqueous CMC solution is a function of the molecular weight of the CMC employed. Consequently, hereinafter, the terms "molecular weight" and "viscosity" will be used interchangeably unless the particular context indicates otherwise.
The cellulose employed as a furnish for the carboxymethylation reaction used to produce CMC is normally of a relatively high molecular weight. It is desirable to avoid molecular weight reduction during the carboxymethylation because of resultant yield losses. Consequently, when low and medium molecular weight CMC products are sought it is desirable that the molecular weight reduction take place only after high molecular weight CMC has been produced.
It is known in the prior art that the molecular weight of CMC can be reduced by the irradiation of relatively high molecular weight material. The reduced molecular weight product, however, is unstable and undergoes further molecular weight reduction as is taught in U.S. Pat. No. 4,051,306 ('306). In '306 the continued degradation is seen as an advantage, but in most cases the continued degradation is seen as a drawback, for it means that the viscosity of solutions made from irradiated CMC will depend on the time span between the time the CMC is irradiated and the time the CMC is used to make the solutions. The viscosity of such solutions decreases as a function of the time the CMC is stored after irradiation because the CMC continues to degrade.
U.S. Pat. No. 3,108,890 teaches a method for preventing downward viscosity drift of solutions of irradiated cellulose ethers after the solutions are prepared. This is accomplished by adding sufficient alkalizing compound to the solution so that the pH is from about 5.5 to 11.5.
The object of the present invention is to prevent molecular weight change of irradiated CMC during storage. By the process of this invention, the molecular weight can be controlled so as to provide CMC having a stable, desired, predetermined molecular weight. Equal amounts of stabilized CMC will produce solutions having essentially the same viscosities regardless of the length of time between irradiation and solution preparation.